1. Field of the Invention
The present invention relates to a wiring structure of an integrated circuit that is formed on a substrate having an insulating surface by using thin film transistors (hereinafter called TFTs). The invention also relates to a wiring structure of a liquid crystal display device of a peripheral circuits integration type that is formed on a substrate having an insulating surface by using TFTs.
2. Description of the Related Art
A technique is known in which a silicon film having crystallinity is formed on a glass substrate or a quartz substrate and a TFT is formed by using the silicon film thus formed. This type of TFT is called “high-temperature polysilicon TFT” or “low-temperature polysilicon TFT.”
The high-temperature polysilicon TFT is formed on a highly heat-resistant substrate such as a quartz substrate because a crystalline silicon film to constitute an active layer is obtained by heating at 800° C. to 900° C. On the other hand, the low-temperature polysilicon TFT is formed on a substrate that is relatively low in heat resistance such as a glass substrate by a process of less than 600° C.
The high-temperature polysilicon TFT has advantages that TFTs similar in characteristics can easily be integrated on a substrate, and that it can be manufactured by utilizing various process conditions and manufacturing apparatuses of the conventional IC processes. On the other hand, the low-temperature polysilicon TFT has an advantage that a glass substrate can be used which is inexpensive and can easily be increased in size (large-area substrate).
According to the current technologies, there are no large differences in characteristics between the high temperature polysilicon TFT and the low-temperature polysilicon TFT. Both types of TFTs provide mobility values of about 50 to 100 cm2/Vs and S-values of about 200 to 400 mV/dec (VD=1 V).
Techniques of producing a liquid crystal display device in which integrated circuits, an active matrix circuit, and peripheral circuits for driving it are formed on the same substrate (what is called a peripheral circuits integration type liquid crystal display device) are now being studied.
However, the characteristics of the conventional high-temperature polysilicon TFT and low-temperature polysilicon TFT are much poorer than those of the MOS transistor that using a single crystal silicon wafer. In general, the MOS transistor using a single crystal silicon wafer provides an S-value of 60 to 70 mV/dec.
Further, in each of the high-temperature polysilicon TFT and the low-temperature polysilicon TFT according to the current technologies, because of low mobility, the driving frequency of the TFT itself is obliged to be less than several megahertz.
For example, where peripheral circuits of a liquid crystal display device are formed by using high-temperature or low-temperature polysilicon TFTs, it is impossible to directly input, to drive the TFTs, a clock signal or a video signal of more than tens of megahertz that is necessary for display.
For the above reason, a plurality of wiring lines (interconnections) are used to transmit clock signals or video signals and the clock signals or video signals are supplied to the TFTs in such a manner as to be reduced in frequency (called divisional driving). For example, a 10-MHz frequency of an original clock signal is divided into 2.5 MHz by using four wiring lines and the respective TFTs are driven at this low frequency. This increases the number of wiring lines and the number of TFTs, resulting in problems such as an increased installation area.
On the other hand, the present inventors have developed a TFT which exhibits performance that is equivalent to that of the MOS transistor using a single crystal silicon wafer though it uses a crystalline silicon film.
This type of TFT uses a crystalline silicon film having a crystal structure that is continuous in a predetermined direction, for instance, in the source-drain direction as well as having grain boundaries extending in the same, predetermined direction.
This type of crystalline silicon film is obtained by introducing a very small amount of a metal element (for instance, nickel) for accelerating crystallization into an amorphous silicon film, then heating the amorphous silicon film at 500° to 630° C. (for instance, 600° C.) to cause lateral crystal growth, and thereafter forming a thermal oxidation film.
Having much superior characteristics such as an S-value of smaller than 100 mV/dec and mobility of higher than 200 cm2/Vs, this type of TFT, in itself, can be driven at tens to hundreds of megahertz or even higher frequencies. By using this type of TFT, TFTs capable of being driven at high speed can be integrated on a large-area substrate.
As a result, not only can circuits having much superior performance be obtained but also the numbers of thin-film transistors and wiring lines necessary for driving can be reduced to a large extent from the conventional cases, thereby greatly contributing to miniaturization and increase in the degree of integration of devices.
However, where an integrated circuit is formed by using TFTs over such a large area as a several centimeter square to a tens of centimeter square as in the case of the peripheral circuits integration type active matrix liquid crystal display device, the rounding of high-frequency signals that are transmitted by wiring lines becomes a very serious problem when the integrated circuit is driven at a high frequency such as tens to hundreds of megahertz or a even higher frequency.
This problem will be described below for peripheral circuits of a liquid crystal display device. FIG. 5 is a top view of a peripheral circuits integration type active matrix liquid crystal display device.
As shown in FIG. 5, an opposed substrate 902 having an opposed electrode (not shown) on its inside surface is opposed to a substrate 901 with liquid crystal material (not shown) interposed in between.
A data lines (source lines) driving peripheral circuit 903, a scanning lines (gate lines) driving peripheral circuit 904, and an active matrix display section 905 in which respective pixels are provided with pixel electrodes and switching TFTs that are connected to the respective pixel electrodes are provided on the substrate 901.
A flat cable 906, which extends from external circuits to supply signals to the liquid crystal display device, is electrically connected to peripheral wiring lines 907 at an end portion of the substrate 901. The peripheral wiring lines 907 are connected to wiring lines 908 and 909 in the peripheral circuits 903 and 904. The peripheral wiring lines 907 and the wiring lines 908 and 909 in the peripheral circuits 903 and 904 are arranged parallel or approximately parallel with each other.
The wiring lines 907 to 909 are formed as thin films of a conductive material such as aluminum at the same time as the TFTs of the peripheral circuits 903 and 904 and the active matrix circuit of the display section 905 are formed.
Part of the wiring lines 907 to 909 are used for transmitting a signal of a very high frequency, for instance, more than 10 MHz. Typical examples of those wiring lines are a video signal line for transmitting a video signal and a clock signal line for supplying a clock signal.
In general, the clock signal frequency amounts to about 12.5 MHz in the case of VGA (640×480×3 (three colors of RGB) pixels), and the video signal frequency increases with the clock signal frequency, i.e., as the image resolution becomes higher.
In particular, in the peripheral circuits integration type liquid crystal display device, the peripheral circuits 903 and 904 for driving the display section 905 of a several centimeter square to a tens of centimeter square are usually provided along sidelines of the display section 905 and hence have a length of several centimeters to tens of centimeters along the sidelines.
Each of the peripheral circuits 903 and 904 has wiring lines that extend from one end to the other of the circuit. The clock signal line and the video signal line are examples of such wiring lines. Such wiring lines has a length of several centimeters to tens of centimeters inside the peripheral circuits 903 and 904.
The electric resistance of each of such long wiring lines becomes very high even if it is made of a material having high electric conductivity, such as aluminum.
The peripheral wiring lines 907 for transmitting signals from the flat cable 906 to the peripheral circuits 903 and 904 are also such that the line width is tens to hundreds of micrometers and the length is several centimeters or more, even tens of centimeters in some cases.
In view of the length of the peripheral wiring lines 907 and the length of the wiring lines 908 and 909 in the peripheral circuits 903 and 904, it is understood that signals are transmitted by so long wiring lines as never occur in the scale of conventional IC chips.
On the other hand, capacitance coupling likely occur in the parallel-arranged wiring lines when a high-frequency signal is applied thereto because they are distant from each other by merely tens to hundreds of micrometers.
Further, in the liquid crystal display device, the opposed electrode (not shown) is provided on the entire surface of the opposed substrate 902. From the viewpoints of protecting the peripheral circuits 903 and 904 and simplifying the manufacturing process, it is a common design to provide not only the display section 905 but also the peripheral circuits 903 and 904 and the peripheral wiring lines 907 on the surface that confronts the opposed substrate 902. Therefore, the opposed electrode confronts the peripheral wiring lines 907 and the wiring lines 908 and 909 in the peripheral circuits 903 and 904, and hence capacitance coupling may occur between the opposed electrode and the above wiring lines.
The capacitances formed between wiring lines or between wiring lines and the opposed electrode (provided on the inside surface of the opposed substrate 902 that confronts the substrate 901 via the liquid crystal) and the high. resistance of each wiring line cause deterioration, i.e., rounding, of a transmission signal waveform. That is, a signal that is transmitted by a wiring line, even if it has a good shape (for instance, a rectangular shape) at the input stage, is more rounded (the rising position of the waveform is delayed or the waveform is disordered) as it reaches the end of the wiring line.
If a signal waveform is rounded to a large extent, a delay may occur in the operation timing of a circuit or erroneous video information is transmitted to pixels, possibly resulting in erroneous operation or a disordered image.
This problem becomes more serious as the size of the display section 905 increases or the driving frequency is increased by increasing the display resolution.
Among the peripheral circuits 903 and 904, the rounding has a great influence on, i.e., is a serious problem in the circuit 903 for driving the data lines (source lines) because it is supplied with high-frequency signals of tens to hundreds of megahertz.
At present, integrated circuits in the form of a chip that uses a single crystal silicon wafer are also common that operate at a driving frequency of tens to hundreds of megahertz. However, in such cases, since the entire integrated circuit is accommodated in a chip of an about 1-to-2-cm square, wiring lines are short and hence the rounding is less serious than in the large-area liquid crystal display device.
To reduce the capacitance between wiring lines, it is necessary to increase the distance between the wiring lines and decrease the dielectric constant of the region between the wiring lines.
However, if the distance between wiring lines is increased, the area necessary to accommodate the wiring lines and a circuit that uses the wiring lines increases, resulting in an increase in the size of the entire device. Thinning the width of wiring lines is not favorable either because the electric resistance increases though the distance between the wiring lines decreases.
The distance between the wiring lines and the opposed electrode is relatively small (the interlayer insulating film is 1 to 2 μm thick and the liquid crystal layer is 3 to 8 μm thick, and hence the total thickness is about 10 μm). However, the thickness of the liquid crystal material layer, i.e., the cell gap, cannot be increased for the optical reasons. It is difficult to increase the distance between the wiring lines and the opposed electrode sufficiently to obtain a desired reduction in capacitance by increasing the thickness of the interlayer insulating layer.
As described above, it is difficult for the current technologies to effectively reduce the capacitance between wiring lines.
One may think that the electric resistance of wiring lines can be reduced by widening or thickening the wiring lines. However, thickening the wiring lines is unfavorable because it makes hillocks to occur more easily due to heating in a manufacturing process and hence short-circuiting comes to occur more easily between wiring lines that cross each other via the interlayer insulating film.
On the other hand, thickening the wiring lines is not preferable either, because it makes hillocks be caused more easily by a heat treatment in a manufacturing process as well as makes the wiring lines be short-circuited more easily with other wiring lines that cross the former wiring lines with an interlayer insulating film interposed in between.